The quality I hold in highest regard is uniqueness. I will never review the Starbucks on South U, because it's Starbucks. If you've been to one, you've been to them all. Ann Arbor is home to few chain restaurants (though when it is host to a chain, it feels the need to play host to 87 of them, e.g. Jimmy John's), and I think it is an indication that we haven't yet given up our 'bastion of individualists' title. If a restaurant is simply a clone of Ruby Tuesdays, Bennigan's, etc. Or if it's a fairly run-of-the-mill bar, I will rate it lower than I would a more unique business.
The next most important quality is pricing. If someone plans on charging me $6 for a domestic draft, $9 for a movie ticket in an uncomfortable seat, or $12 for a standard hamburger when there is equal or better food/drink/tickets elsewhere, I'm going to dock them. It's not so much a matter of me giving the best ratings to the cheapest businesses, but I will give the best ratings to the places that have better than average quality for how much you are paying.
The other big quality that determines the rating I will give a business is the atmosphere. While this plays into the two above listed traits, I feel I need to make clear that it is, in an of itself, distinct. The atmosphere is affected by the typical patrons (students? professors? Yspians?), the quality of the staff (are they nice? is service fast or slow?) and, for lack of a better term, the Feng Shui of the locale. A restaurant where you have to sit around for an hour to get your food under bright florescent lights while shady homeless guys hassle you for change is going to get more than a couple stars taken off of it's ranking.
I will try to rate places with consistency in category. Sports bars should have the same qualities as lounges. Short order diners shouldn't be graded on their visual appeal like a sit down restaurant should. I will make my best efforts to compare apples to apples, and feel free to call me out if I slip.
Finally, the scale I will use is from 1 to 5 stars. Five star businesses are places that you won't find anywhere else, are worthy of going again and again, and won't destroy your wallet. One stars are places I hope you never go to and I seriously regret any time I have ever gone there. Anything three stars an above you should make an effort to go to at least once, to complete your Ann Arbor experience.